BJT saturated switch
Base drive, GPIO loading and switch dissipation using an explicit forced beta.
Development preview. Check the assumptions below before relying on a result. Calculations run in your browser.
How it calculates
Base resistor: RB = (Vdrive − VBE) / (IC/βforced + VBE/RBE)
Assumptions
- Use VCE(sat) and VBE(sat) at the selected collector/base currents from the device datasheet. GPIO high voltage must be specified at this load.
- Inductive loads need a separate clamp calculation (TR-10).
Worked example
These are the calculator's default inputs. Change any input above to recalculate.
Inputs
- Load supply
- 12 V
- GPIO high voltage
- 3.3 V
- Collector current
- 0.1 A
- Forced beta
- 10
- Minimum active-region beta
- 50
- Saturated base-emitter voltage
- 0.85 V
- Saturation voltage
- 0.2 V
- Base-emitter pull-down
- 10000 Ω
- GPIO source-current limit
- 0.008 A
Results
- Maximum base resistor
- 242.935 Ω
- Transistor base current
- 0.01 A
- Required GPIO current
- 0.010085 A
- Base-resistor power
- 0.0247082 W
- Transistor dissipation
- 0.0285 W
- Load voltage
- 11.8 V